The present invention relates to fast scanning magnetic resonance imaging. It finds particular application in conjunction with medical diagnostic imaging procedures in which data is concurrently acquired for multiple images with different T1/T2 weightings and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with fast scanning sequences for generating a single image and for non-medical applications.
In many magnetic resonance imaging sequences, the repeat time TR is selected to be substantially longer than the T2 relaxation time of the material being imaged. This permits the transverse magnetization which is excited in each repetition of the sequence to die out before the next repetition, of course, if the repeat time is shortened, then the time required to collect a full set of data is shortened proportionately. However, when the repeat time is shortened to be comparable to or less than the T2 relaxation time, transverse magnetization excited in one repetition persists or carries over into the next repetition.
Various fast scanning sequences have been developed in which the repetition time TR is less than the T2 relaxation time of the material being imaged. In each repetition of the sequence, there is a freshly excited or first generation resonance signal component and older persisting (second and higher generation) residual magnetization signal components which carry over from the preceding one or more repetitions. When the freshly excited and persisting residual resonance magnetization signal components are allowed to coincide, the resultant images are difficult to interpret. The different signal components have had different amounts of time (TE) in which the signal may decay, and thus one cannot expect to see the typical contrast associated with any one TE. Worse yet, the residual signal components have different histories of phase evolution than the freshly excited component. In particular, the different histories involve different phase accumulation, and so the residual components may not reconstruct properly at all, or they may reconstruct with minor phase errors causing them in some places to add constructively to the main component, and in other places to add destructively.
One can run a generic field echo scan with a very short TR and with a small flip angle. The freshly excited and residual signals from several repetitions can be permitted to overlap without expressly worrying about alignment in the time of the echoes, refocusing phase echoes, or inducing dephasing. In this manner, the freshly excited and residual magnetization signals add together to produce a stronger resonance signal for imaging. However, this technique is prone to serious banding artifacts and interference patterns. Because the single resultant image is actually a complicated superposition of images, its contrast is hard to interpret.
In one technique for improving the resultant images, untwisting or phase decoding gradients are applied at the end of each repetition to erase the phase history of the residual components. In this manner, the residual magnetization and the freshly excited magnetization signal components have the same phase encoding. When the residual and freshly excited magnetization signal components are caused to refocus concurrently into echoes, their contributions add improving the signal-to-noise ratio. This enables a steady-state to be reached in which the magnetization in each repetition has the same ratio of freshly excited to residual signal, one of the drawbacks to this technique is that it is prone to banding artifacts and has a complicated contrast. A source of the banding is phase dispersion from static field inhomogeneities, chemical shift, motion, or the like, which are phase-producing mechanisms not compensated for by the phase untwisting gradient waveforms.
Another approach is to spoil the residual signal from the preceding cycles. A constant gradient section is used in each repetition that is not refocused. This spoils the residual signal components and averages the phase dispersion within each voxel. The image intensity of a steady-state type scan varies as a function of the evolving phase dispersion. However, this evolving dispersion effect is reduced or eliminated by causing each voxel to contain one complete cycle of phase variation or an integer number of phase wraps. Leaving a constant amount of gradient without rephasing in each repetition interval insures that all voxels exhibit a comparable, average phase dispersion. However, this technique may still yield some banding artifacts and striping. It does not make use of all available signal. Rather, the signal-to-noise of the total scan is lower than other techniques described above.
Another technique is to spoil the residual magnetization signal components with varying gradients. When the amplitude of the spoiling gradient is altered from repetition to repetition, a spoiling effect is achieved under conditions where constant amplitude spoiler gradients would just cancel out. The spoiler gradients are applied along coherency pathways with RF spin echoes or with RF stimulated echoes. The spoiler gradients either increase progressively in steps with each repetition or alternate polarity. However, these techniques do not make use of the total available signal. Stepping the spoilers over long ranges which do not repeat is impractical, especially with shorter repetitions. Often the hardware to support large gradients is not available. Stepping spoiler gradients through short patterns breaks up the shorter coherence pathways but allows longer RF echoes to be refocused. Using large sections of gradients which alter significantly from repetition to repetition may induce detrimental gradient eddy current effects which distort images and destroy steady-state effects.
There is a class of techniques in which the RF pulse is modified from repetition to repetition. The phase modification is used in one of several ways to reduce artifacts. For example, the phase of the RF pulse is indexed in steps which vary throughout the duration of the scan. In another technique, two scans are taken. In one scan, the RF phase is held constant and in the other the RF phase is alternated. The images obtained from two scans are selectively combined such that regions of one image with banding artifacts are replaced with artifact-free portions of the other image. In another related technique, the RF transmit phases are stepped and different transforms are performed to separate the various echoes. The other RF modified techniques also have the disadvantage of requiring longer scan times, or the disadvantage of greatly reducing resolution to achieve the same short total scan time. In the first technique in which the phase step varies, less than all of the total available signal strength is used. Although this technique reduces artifacts, residual artifacts are still found.
In other techniques, the residual and freshly excited signal components are caused to generate echoes which are offset in time. In this manner, the echoes from each component occur one after the other and are acquired in succession. This enables the data from each echo to be handled separately to generate multiple images or a single image which is free of residual magnetization contribution. However, because numerous echoes occur in a very short interval, the sampling time accorded for each is relatively small. This small sampling window decreases the signal-to-noise. Moreover, for a given resolution and a given field of view, more total gradients are required (particularly the read gradient) than in the other methods discussed above. In order to be practiced on currently available scanners, it is normally necessary to compromise either the repeat time, the resolution, or the field of view.
The present invention contemplates a new fast scanning technique which overcomes the above-referenced problems and others while generating one or more artifact reduced images.